If Amazon switched over to ePub, it would also make it easier for them to support DAISY format ebooks, which are marked up with XML (more information here:
http://www.daisy.org/about_us/dtbooks.asp).
The Internet Archive converts all their ebooks (1.6 million and counting, all free) to ePub as well as DAISY. I saw a demo of the DAISY reader at one of their events recently and it was very promising.